<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <title>Title</title>
</head>
<body>
  <span class="a1">a1</span>
  <span class="a2">a2</span>
</body>
</html>
<style>
  .a1:hover{
    display: none;
  }
  .a1:hover+.a2{
    font-size: 20px;
    color: red;
  }
</style>
